Children in England are three months behind in their studies after lockdown, with boys and poor pupils worst hit, suggests a survey of teachers by an educational research organisation.
The learning gap between rich and poor pupils grew by almost half between March and July, the National Foundation for Educational Research has found.
The authors also warn a quick catch-up is unlikely.
The government says children must not lose out because of coronavirus.
The new term begins in England and Wales this week, after the disruption caused by the Covid-19 pandemic. Schools are already back in Scotland and Northern Ireland.
The National Foundation for Educational Research's survey questioned a weighted sample of almost 3,000 heads and teachers in about 2,200 primary and secondary schools across England.
The research was carried out just before the end of term in July - and showed how much children had fallen behind by the end of the last school year.
Almost all the teachers questioned (98%) said their pupils were behind the place in the curriculum they would normally expect for the time of year.
Overall, teachers said they had covered just 66% of their usual curriculum by July, putting pupils three months behind in their learning.
The researchers also found:
- boys are further behind in the curriculum than girls
- the learning gap for poorer pupils has widened by at least 46%
The National Foundation for Educational Research (NFER) report found teachers in the most deprived schools (those with the greatest proportion of pupils on free school meals) were more than three times as likely (53%) to say their pupils were at least four months behind, compared with those in the wealthiest schools (15%).
Even for those pupils who had places in school last term, attendance was poor - only 56% of eligible pupils actually went back amid safety concerns from parents, the report reveals,
Almost three quarters of the teachers questioned thought they were unable to teach to their usual standard under the coronavirus regulations.
Overall, teachers estimate 44% of their pupils are in need of intensive catch-up support, says the report, with teachers in the most deprived schools (57%) more likely to believe this than those in the wealthiest schools (32%).
NFER chief social scientist, Dr Angela Donkin, welcomed the government's National Tutoring Programme but questioned "whether the scale will be sufficient to meet the high demand for those requiring intensive support".
Almost all the school leaders questioned (90%) predicted they could manage to open to all pupils safely, however more than three quarters (78%) expressed concerns, with many saying additional funding would be needed for more staff, cleaning and protective equipment.
Separately, head teachers and teachers criticised the government for "last-minute" guidance on what to do during virus outbreaks and local lockdowns, which was published on Friday.
In the NFER report, teachers urged better planning for further lockdowns, and called for more and better IT equipment for pupils and staff. More than a quarter of pupils (28%) were reported to have no access to a laptop or computer at home.
The authors said it was encouraging that the government was offering laptop and internet connections for disadvantaged pupils but that "a much swifter dispatch of devices" was needed, as well as more training for teachers.
Other recommendations included:
- more safety reassurance for parents
- better support for schools to bring in "non-attending" pupils
- more money for enhanced cleaning and extra staff to ensure social distancing
- Ofsted to modify its expectations of schools during social distancing
- acceptance that catch-up will be a long-term endeavour
"Whilst it is crucial that children catch up, we should not assume that teachers will immediately be able to deliver the same quality of teaching at the same speed, as before the pandemic," said Dr Donkin.
Mary Bousted, joint general secretary of the National Education Union, asked the government to hire "qualified teachers not currently in post" to help reduce class sizes, which would in turn "provide educational catch-up and ensure safety for all".
Dr Bousted also urged a more flexible approach to exams next year, "one which learns from the mistakes of this year".
Shadow education secretary Kate Green said on Monday that students starting Year 11 and 13 in September had "a mountain to climb", having missed months of schooling.
Labour leader Sir Keir Starmer called on Mr Williamson to face parliament to explain "how he will protect" children's futures.
"He needs to explain how he will make up for the damage already done, bring pupils up to speed and mitigate against the ongoing risk from the pandemic," Sir Keir added.
Paul Whiteman, general secretary of the National Association of Head Teachers (NAHT), called the NFER report "another alarm bell" for the government.
The head teachers' union is also calling for a temporary ban on fining parents and guardians if their children do not return to school.
Mr Whiteman said: "If you are a parent and you are worried about safety, a fine is unlikely to make you feel any safer."
The Department for Education has said fines for school absences would only be used as a "last resort" in England.
In a statement, the department added: "Throughout the pandemic we have invested in remote education, providing devices, routes and resources for the children who need them most and why our £1bn Covid catch-up package will tackle the impact of lost teaching time, including targeted funding for the most disadvantaged students."